Commentary for Leviticus 18:5

'ושמרתם את חקתי וגו‎ YE SHALL THEREFORE KEEP MY ORDINANCES — [This is not a mere repetition of v. 4 but is intended] to include other details of the laws mentioned in this chapter which Scripture does not expressly mention. Another explanation is: Scripture makes this repetition in order to attach the commands of “observing" (שמירה) and "performing" (עשיה) to the חוקים, and the commands of שמירה and עשיה to the משפטים, for in v. 4 it uses the term עשיה only in connection with the משפטים, and that of שמירה only with reference to the חוקים (Sifra, Acharei Mot, Section 8 10).

ושמרתם את חקותי, "You shall keep My statutes, etc." Why did the Torah have to repeat in this verse almost word for word the same directive it had written in the previous verse? Perhaps we may best explain this on the basis of Sanhedrin 74 where the Talmud explains the words וחי בהן, "so that he may live by them," as meaning "so that he will not die by them." If a Jew is forced to violate one of G'd's commandments, he should rather violate such a commandment than make a martyr of himself. The Talmud adds that there are three exceptions to this rule, idol-worship, forbidden sexual relations, and killing an innocent person. If a Jew is threatened with death if he does not violate any of these three commandments he must choose death rather than violate any of these commandments. You will observe that in verse 4 no mention is made of וחי בהם, that one should prefer life over martyrdom, whereas in our verse the Torah added the words וחי בהם. It follows that the commandments mentioned in verse 4 are those for which a person must be prepared to martyr himself and to sanctify the name of G'd by laying down his life.

וחי בהם. However, if the Israelite does not observe these statutes the souls violating them deliberately will be cut off from membership in their nation, i.e. will lose their life in the hereafter (verse 29)